Glaciers begin to form when snow remains in the same area year-round and transforms into ice. New layers of snow bury and compress the previous layers. The compression forces the icy snow to re-crystallize, forming grains similar in size and shape to cane sugar. Gradually the grains grow larger and the air pockets between the grains get smaller and more packed, increasing its overall density. At 20 meters (65 feet) the sheer weight of the mass causes the lowest portions to deform into sliding layers, and the glacier begins to flow. Learn more through this interactive.